U.S. stocks ended down on Tuesday in a retreat from the previous session's sharp rally as energy shares declined and the dollar edged up, but the S&P 500 and Nasdaq registered their ninth straight quarterly rise. The S&P's 500 quarterly winning streak was its longest since 1998, while the Nasdaq's was its longest ever. The Dow registered a slight loss for the quarter. The S&P Energy index declined 0.9 percent, while shares of Exxon Mobil Corp fell 0.7 percent to $85.

A plan to salvage RadioShack Corps business by co-branding most of its 1,740 surviving stores with cellular phone provider Sprint Corp earned U.S. bankruptcy court approval on Tuesday, ending four days of contested court hearings. The stores are what survived of more than 4,000 outlets after RadioShack went bankrupt in February. Judge Brendan Shannon, in Delaware bankruptcy court, approved a sale of the stores to the Standard General hedge fund, which plans to keep most of them open under a deal in which Sprint will occupy one-third of each space.

By Barani Krishnan NEW YORK (Reuters) - Oil fell for a third straight session on Tuesday, with Brent crude tumbling 12 percent for the month, as world powers entered into intense negotiations with Iran for a nuclear deal that could bring more of its oil to an oversupplied market. In Tuesday's session, prices ended off the day's lows as the United States, Britain, France, Germany, Russia and China faced difficulty in reaching a preliminary nuclear accord with Iran before a Tuesday midnight deadline at the talks in Lausanne, Switzerland. Brent oil settled down $1.18, or 2.1 percent, at $55.11 a barrel, after falling to $54.72 during the session. In the past three sessions, oil lost more than 7 percent on heightened fears that a nuclear deal for Iran would raise the global glut in crude.

An early example to watch is Monsanto Co , which reports results on Wednesday, April 1 for its fiscal quarter ended Feb. 28. On Jan 7, before the dollar's biggest moves of the quarter, the company said currency issues would reduce its 2015 earnings by between 15 cents and 20 cents, similar to 2014, when the dollar rose 12.8 percent against a basket of major currencies. At Monsanto, for example, other factors such as an improving outlook for corn planting may help offset currency losses, said Paul Massoud, analyst at Stifel. The rapid devaluation of nearly every foreign currency versus the U.S. dollar is not news, yet surprisingly only about 30 percent of the sell-side analysts who follow Red Hat have updated their models at current rates, said Charles Peters, the CFO.

Warren Buffett, the billionaire chief executive officer and chairman of Berkshire Hathaway Inc, said Tuesday that an exit by Greece from the euro zone could be constructive for the region. "If it turns out the Greeks leave, that may not be a bad thing for the euro," Buffett told cable television network CNBC. He said a Greek exit from the euro zone could lead member countries to come to better agreement about fiscal policy. "If everybody learns that the rules mean something and if they come to general agreement about fiscal policy among members or something of the sort, they mean business, that could be a good thing," Buffett said.

U.S. consumer confidence rebounded strongly in March amid optimism over the labor market while house prices increased in January, hopeful signs that a recent sharp slowdown in economic activity was probably a blip.

European Union antitrust regulators have asked Luxembourg for information on its tax rulings for McDonald's after labor unions accused the U.S. fast food chain of avoiding taxes, a person involved in the issue said on Tuesday. The move, which neither the EU nor Luxembourg would immediately confirm, falls well short of the proceedings the EU has instigated against other U.S. multinationals in Luxembourg and elsewhere, where Brussels believes unfairly low taxation may have distorted competition. Unions from the United States and Europe and British-based charity War on Want last month urged the European Commission to investigate what they said involved about 1 billion euros ($1.07 billion) in tax between 2009 and 2013. "The Commission has sent a letter to Luxembourg asking them to clarify the facts," the source told Reuters.

By Dan Levine SAN FRANCISCO (Reuters) - Ellen Pao faces an uphill battle should she choose to appeal her defeat last week in a gender discrimination lawsuit against former employer Kleiner, Perkins, Caufield & Byers, the Silicon Valley venture capital firm. Not only did a San Francisco Superior Court judge side with Pao on some key evidentiary disputes in the run-up to trial, but employers have been highly successful litigating in the California appeals court where Pao's case would land. Conversely, the court - which covers San Francisco and 11 other Northern California counties - handed victory to employers in more than half of the cases they lost in the lower courts, reversing 10 of 18 cases. Pao's lawsuit helped spark a wide-ranging debate about the treatment of women in Silicon Valley, which has continued after a jury last Friday decided she had not proved Kleiner broke the law when it passed her over for promotion.

DETROIT (Reuters) - Warren Buffett's Berkshire Hathaway Inc wants to purchase more U.S. auto dealerships to expand the company's automotive dealer venture, Larry Van Tuyl, chairman of Berkshire Hathaway Automotive, told CNBC on Tuesday. Van Tuyl, who appeared on CNBC with Buffett, said that the group will look for high-volume dealerships to add to the 81 dealerships it owns in 10 states. (Reporting by Bernie Woodall; Editing by Meredith Mazzilli)